In a bleached pulp mill operation, in which bleach plant spent chemicals are introduced to the spent pulping liquor recovery and regeneration operation to eliminate the polluting effects of such chemicals, chemicals which normally are purged from the pulp mill by the discharge of bleach plant spent chemicals and which are not chemically affected by the recovery and regeneration operation accumulate in the recycling pulping liquor.
The major one of these accumulating chemicals is sodium chloride. Techniques for the removal of sodium chloride from the recovery and regeneration operation are described, for example, in U.S. Pat. Nos. 3,746,612, 3,950,217, 3,909,344, 3,986,923, 3,945,880 and 3,954,552.
Wood contains minor quantities of potassium salts, typically about 0.05 to 0.2% K.sub.2 O, corresponding to about 2 to 7 lbs. of potassium per ton of pulp (assuming a pulp yield of about 45%). Usually such quantities of potassium material are purged from the pulp mill by way of discharge of bleach plant chemicals, liquor spills, and furnace flue gases. However, with the implementation of the closed cycle concepts outlined above, and the tightening up of pulp mill procedures resulting in the elimination of liquor spills and the discharge of particulates with furnace gases, the potassium values tend to accumulate in the recovery and regeneration cycle.